Multiple Sclerosis (MS) is an autoimmune disease of the central nervous system that results in multi-focal demyelination of axons and associated axonal damage. Genetic linkage analysis has identified a number of candidate loci associated with MS, but non-MHC genes having a strong association have yet to be identified. The animal model for MS, Experimental Autoimmune Encephalomyelitis (EAE), is a T cell mediated disease. [31,6N-acetylglucosaminyltransferase V (Mgat5), an enzyme in the Asn (N)-Iinked protein glycosylation pathway, is a potent negative regulator of T cell activation and autoimmunity in mice. Mgat5 deficiency lowers T cell activation thresholds by directly enhancing TCR clustering and signaling at the site of antigen presentation. Mgat5-/- T Cells hyperproliferate in response to TCR agonists and Mgat5 deficient mice display increased susceptibility to EAE and develop kidney autoimmune disease. The GIcNAc [31,6branching initiated by Mgat5 is preferentially extended by two or more N-acetyllactosamine units, the ligand.for galectins, a family of carbohydrate binding proteins that regulate T cell signaling, proliferation and apoptosis. TCR associated Mgat5 modified glycans are bound to galectin-3, participating in a putative cell surface galectin-glycoprotein lattice that inhibits TCR recruitment to the site of antigen presentation. To further elucidate the role of Mgat5 and the galectin-glycoprotein lattice in the regulation of T cell function and autoimmunity, three specific aims are proposed. Specific Aim #1 will identify additional protein and carbohydrate components of the T cell galectin-glycoprotein lattice and their relevance to T cell function and adhesion. T cells isolated from mice with targeted mutations in glycosylation will be analyzed for alterations in cell function and receptor - galectin interactions by florescence microscopy, co-immunoprecipitation, mass spectroscopy, signaling and proliferation assays. Specific Aim #2 will explore the regulation and function of Mgat5 glycans in TH1and TH2 CD4+, CD8+ and memory T cell subsets. The influence of Mgat5 deficiency on the balance between TH1 promotion and TH2inhibition of autoimmune disease will be determined. Specific Aim #3 will use EAE induction by adoptive transfer to determine the roles of altered T cell adhesion and non-T cells to the Mgat5 deficient autoimmune phenotype.